Method of preparing cavity envelopes by means of thin-film procedures

ABSTRACT

A cavity envelope is formed by producing a selected shaped volume of diffusant on a substrate, coating the substrate and the diffusant with a porous dielectric, leaching the diffusant from the cavity formed by the substrate and the porous dielectric, creating a vacuum in, or alternatively, placing a gas in the cavity, and then coating the dielectric to eliminate its porosity.

United States Patent 1 [111 3,788,723 Lehrer Jan. 29, 1974 METHOD OF PREPARING CAVITY [56] References Cited 5232 33? MEANS OF THIN-FILM UNITED STATES PATENTS 714,233 11/1902 Plancon 313/220 UX Inventor; I. Lehrer, Los Altos 3,650,003 3/1972 Toyoshima 29/423 [73] Assignee: Fairchild Camera and Instrument Primary Examiner- Roy Lake g g Syosset Long Island Assistant Examiner-.1. W. Davie Attorney, Agent, or Firm-Roger S. Borovoy; Alan H. [22] Filed: Apr. 24, 1972 MacPherson [21] Appl. No.: 70,102 [57] S RA 62 Related Application Data A cavity envelope is formed by producing a selected 1 of 723,706 Apnl 1968 shaped volume of diffusant on a substrate, coating the abandoned substrate and the diffusant with a porous dielectric, leaching the diffusant from the cavity formed by the g 316/ substrate and the porous dielectric, creating a vacuum [58] Fieid 315/2 m, or alternatively, placing a gas in the cavity, and

then coating the dielectric to eliminate its porosity.

8 Claims, 6 Drawing Figures METHOD OF PREPARING CAVITY ENVELOPES BY MEANS OF THIN-FILM PROCEDURES This is a division of US. Patent application Ser. No. 723,706, filed Apr. 24, 1968 now abandoned.

BACKGROUND OF THE INVENTION Field of the Invention This invention relates to the cavity envelopes and, in particular, to a method of preparing such envelopes.

Often it is necessary to enclose a gas or create a vacuum inside an envelope as, for example, in a vacuum tube or a neon bulb. Commonly this is done by preparing a glass envelope, evacuating the envelope, placing the gas in the envelope and sealing the envelope. Unfortunately, using presently known techniques there is a minimum size of glass envelopes beneath which envelope tolerances and cost of fabrication are no longer satisfactory.

BRIEF SUMMARY OF THE INVENTION This invention, on the other hand, makes possible the manufacture of cavity envelopes several orders of magnitude smaller than heretofore obtained. Using the method of this invention, envelopes as small as 1 mil and 0.25 mils thick have been manufactured with extremely close tolerances. This invention can produce cavity envelopes as small as 5,000 angstroms in diameter. The resulting cavity envelopes are produced economically and quickly.

According to this invention, a selected volume of diffusant is produced in a desired pattern on a substrate by well known thin-film techniques. For the purposes of this specification, a diffusant is defined as including any material which, when dissolved, will pass through a porous medium without destroying the medium. Next, a thin coating of a porous dielectric is placed over the diffusant and the bare substrate. The diffusant enclosed by the porous dielectric and the substrate is then removed by a leaching process using a suitable solvent. By placing the dielectric and the substrate in a vacuum, the cavity left by the diffusant is cleansed of the remaining leaching fluid. If desired, a selected gas is placed in the cavity. A thick coating is then deposited over the dielectric to seal and strengthen it. The resulting gas-filled cavity has extremely accurate dimensions and, if electrodes are provided on the substrate, can be used, for example, as a gas-filled bulb or electron tube.

This invention will be more fully understood in light of the following detailed description taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a substrate with a diffusant and elec- ,tr odes placed thereon;

" 1'. FIG. 2 shows the structure of FIG. 1 with all but a selected volume of diffusant removed;

FIG. 3 shows the structure of FIG. 2 together with a thin layer of porous dielectric placed on top of the substrate, the electrodes, and the diffusant;

FIG. 4 shows the structure of FIGS after the diffusant beneath the dielectric has been leached away;

FIG. 5 shows the structure of FIG. 4 after the cavity left by the diffusant has been left as a vacuum or filled with a gas and a thick coating deposited on top of the dielectric; and.

FIG. 6 shows a three-dimensional cut-away view of a typical cavity envelope produced by the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of this invention can best be described with the assistance of FIGS. 1-5 which show schematically the several steps of this method. In FIG. I, a substrate 1, which can be any rigid, inorganic or organic structure capable of withstanding the subsequent processing, has placed upon it electrodes 3 and 4 and a selected diffusing material 2. Diffusing material 2 is typically a photoresist material, for example, AZ resist supplied by Shipley Company. Diffusant 2 is masked in a manner well known in the art to leave unexposed a small volume of diffusant directly above the interior ends a and b of electrodes 3 and 4 respectively. The masked diffusant is exposed to light, the mask is re moved and the exposed diffusant is developed and removed, again in a manner well known to the art. Left is a small volume 6 of diffusant 2, shown in FIG. 2 centered above the ends a and b of electrodes 3 and 4 and contacting substrate ll between and on both sides of these electrodes. The shape of volume 6 is determined by the mask. If resists other than AZ resists are used, the regions of diffusant left unmasked will depend on whether the exposed or unexposed diffusant is removed by the developing process.

Next, as shown in FIG. 3, a thin film of porous dielectric 5, for example, SiO, is deposited on top of the sub strate l, electrodes 3 and 4, and diffusant 2 by well known vacuum deposition techniques. This film must be porous; a typical thickness to ensure porosity is 1 micron. Other thicknesses of porous dielectric can also be used. But the thickness of dielectric 5 is selected to prevent its collapse when the diffusant 2 is removed from between dielectric 5 and substrate 1, while at the same time maintaining adequate dielectric porosity to solvents.

After the dielectric 5 has been deposited, the diffusant 2 is leached from the cavity 7 (FIG. 4) created by dielectric 5 and substrate 1. The solvent used in the leaching process is, of course, one in which the diffusant is soluble. Appropriate solvents for AZ resists are ketones for example, acetone or alcohols for example, methyl alcohol or isopropyl alcohol. As one bath of solvent becomes saturated with diffusant, a new bath is substituted. Successive baths of solvents are used until the diffusant is substantially removed from cavity 7. The solvents are at room temperature although it is not necessary that they be so maintained.

After the diffusant has been leached from cavity 7 (FIG. 4) some solvent and diffusant remains in the cavity. To remove this residue, substrate 1 and dielectric 5 are placed in a vacuum and cavity 7 is pumped out; that is, the residue is removed from cavity 7.

Next, if it is desired to fill cavity 7 with a gas, substrate l and dielectric 5, hereafter together called a bubble, are placed in a selected gas environment. The gas diffuses through the porous dielectric 5 into cavity 7. A typical gas for use in cavity 7 is neon. When neon is placed in cavity 7, the resulting bubble becomes a neon bulb which, when excited by the correct potential on electrodes 3 and 4, will glow. Other gases, such as argon or mercury vapor, can also be placed in cavity 7.

After the gas has been placed in cavity 7, coating 8, commonly transparent, is deposited on dielectric 5, as shown in FIG. 5, to reduce its porosity. This coating also strengthens dielectric 5 to prevent breakage and can be either the same dielectric as dielectric 5, or a low temperature material such as arsenic glass. Arsenic glass, when heated to approximately 200 C, melts and flows into the porous spaces of dielectric 5 effectively sealing this dielectric.

FIG. 6 shows a three-dimensional sectional view of one possible cavity envelope produced by this invention. Electrode 4 protrudes into the cavity as shown. It is to be understood that more than two electrodes can, if desired, protrude into cavity 7.

In one use of this method, micro-minature neon bulbs were fabricated on a glass substrate. To do this, aluminum electrodes of a selected shape were deposited on the glass by well known vacuum and etching techniques. A diffusant, AZ resist, was then deposited on the glass and aluminum and a circular volume of diffusant 5 mils in diameter and one-half mil thick was formed on the glass by the above described masking and developing, techniques. This volume of diffusant overlapped selected ends of the aluminum electrodes. Next, a one micron layer of SiO, a porous dielectric, was placed over the diffusant, electrodes, and sub strate. The AZ resist was leached from between the dielectric and substrate with two solvents, first acetone and then isopropyl alcohol. This leaching required about 10 minutes. A vacuum of 10 Torr was then used to create a substantial vacuum in (i.e., dc-gas) the volume formerly occupied by the diffusant. The substrate, electrodes, and dielectric were heated to approximately 400C to speed up the de-gassing process. After about minutes of de-gassing the structure was cooled to room temperature, and neon, placed in the vacuum system at a pressure of millimeters of mercury, was diffused for about 15 minutes into the vacuum between the dielectric and the substrate. Following this a 4 micron film of SiO was deposited on the first layer of SiO dielectric to seal the neon in the cavity between the dielectric and the substrate. Upon application of a selected voltage to the electrodes, a cathode dischargev glow was obtained.

It should be understood that any material dissoluble I in any solvent which will not dissolve the dielectric and the substrate can be used as a diffusant in this invention. The term diffusant has been defined to include any such material. In addition, a metal such as aluminum, which reacts with a material such as iodine to produce the compound aluminum iodide All;, which in turn is soluble in an organic solvent, such as carbon disulfide CS can be used as the diffusant. In this case, of course, the electrodes cannot be aluminum. Or, alternatively, a salt such as sodium chloride NaCl or arsenic trisulfide As S soluble in water or ammonium hydroxide respectively, can be used as the diffusant.

Typically, when a non-photoresist material is used as the diffusant, the shaped-volume of diffusant on the substrate needed to form the cavity is obtained in the following manner. First, a photoresist material, usually AZ resist, is placed on the substrate. Then all but the volume of resist needed to form the desired cavity is masked. This unmasked, shaped volume of resist is exposed to light and developed thereby removing it from the substrate, leaving the shaped volume reserved for the cavity. Diffusant, typically the salt NaCl, is placed both over the remaining resist and in the cavity volume left by the developed resist. Lifting the remaining resist 5 from the substrate removes the overlying salt, leaving on the substrate only the salt occupying the desired cavity volume. A suitable porous dielectric, silicon monoxide SiO, or aluminum oxide A1 0 is deposited on the salt. Water is then used to leachthe salt from the cavity. The remainder of the process involves, as described above, creating a vacuum in the cavity, diffusing a gas into the cavity, and coating the dielectric to strengthen and seal it.

Other embodiments of this invention will be obvious in light of this disclosure to individuals skilled in the thin film and vacuum technology arts. In particular, the use of the method of this invention together with other combinations of diffusants, solvents, dielectrics, and substrates to construct a wide variety of shapes, sizes, and combinations of cavity envelopes will be apparent as will also the use of different combinations of electrodes in the cavity envelopes constructed by the method of this invention.

What is claimed is:

1. A method of producing cavity envelopes which comprises: a

forming a plurality of electrodes on a substrate, said electrodes terminating in the region where the cavity is to be formed; producing a shaped volume of diffusant on said electrodes and said substrate; coating said substrate, said electrodes and said shaped volume of diffusant with a selected porous dielectric; leaching said diffusant from between said dielectric and said substrate thereby forming a cavity between said dielectric and said substrate; creating a substantial vacuum in said cavity; and depositing a selected coating on said dielectric to eliminate its porosity after creating said vacuum in said cavity. 2. The method of claim 1 including placing a selected gas in said cavity after creating said vacuum in said cavity.

. 3. The method of claim 1 in which the said diffusant is. a photoresistI material. I

'4. The method of claim 1 in which the step of pro'd u'c- 6. A cavity envelope produced by the method of claim 1.

7. A method of producing cavity envelopes which comprises:

forming a plurality of electrodes on a substrate, said electrodes terminating in the region where the cavity is to be formed;

and said substrate, thereby forming a cavity between said dielectric and said substrate; creating a substantial vacuum in said cavity; and, depositing a coating on said dielectric to prevent passage of material through said dielectric. 8. The method of claim 7 including the step of diffusing a selected gas into said cavity after creating said vacuum in said cavity but before depositing said coating on said dielectric. 

2. The method of claim 1 including placing a selected gas in said cavity after creating said vacuum in said cavity.
 3. The method of claim 1 in which the said diffusant is a photoresist material.
 4. The method of claim 1 in which the step of producing a shaped volume of diffusant on a substrate includes the substeps of: depositing a diffusant on said substrate; masking a volume of diffusant having a predetermined shape to prevent the exposure of said shaped volume to light; exposing all but said shaped volume of diffusant to light; and removing said mask and said exposed diffusant from said substrate.
 5. The method of claim 1 in which said substrate contains a plurality of electrodes terminating in said cavity.
 6. A cavity envelope produced by the method of claim
 1. 7. A method of producing cavity envelopes which comprises: forming a plurality of electrodes on a substrate, said electrodes terminating in the region where the cavity is to be formed; depositing a photoresist diffusant on said electrodes and said substrate; masking a selected portion of said diffusant; exposing the unmasked diffusant to light; removing said mask and said exposed diffusant from said electrodes and said substrate; coating said electrodes, said substrate and said selected portion of said diffusant with a porous dielectric; leaching said diffusant from between said dielectric and said substrate, thereby forming a cavity between said dielectric and said substrate; creating a substantial vacuum in said cavity; and, depositing a coating on said dielectric to prevent passage of material through said dielectric.
 8. The method of claim 7 including the step of diffusing a selected gas into said cavity after creating said vacuum in said cavity but before depositing said coating on said dielectric. 